The present invention concerns a method for casting molten metal into ingots or bars (billets) and an apparatus for the execution of the method.
Normally, molten metal is cast (teemed) in a chill mold, either intermittently into individual ingots or continuously into continuous castings. Most of the surplus heat from the molten metal is dissipated via the chill mold, whereby the metal solidifies. The central portion of the ingot will therefore solidify much later and more slowly than the surface layer, due primarily to the greater heat transmission distance and the higher thermal resistance to the surplus heat in the central portion, whereby temperature gradients are created through the cross-section of the ingot.
Due to the fact that the central portion of the ingot solidifies at a slower rate than the surface, the central portion exhibits a completely different solidification structure and a different chemical composition than the rest of the ingot. Furthermore, porosities, cracks and other flaws form readily in the central portion. These deffects are more serious in higher-alloyed metals, since the solidification gradient is greater.
Compositional differences between precipitated solid phase and molten metal, so-called segregations, can be counteracted on a macro-scale by means of a higher and more uniform rate of cooling of the molten metal and by an equalization of temperature differences by means of stirring in the liquid phase.
Owing to the high segregation tendency of the alloying components in e.g. high-alloyed tool steels, such steels have been cast in relatively small-sized ingots (weighing around 200 kg). In this way, an acceptable carbide precipitation structure is obtained, possessing satisfactory strength properties. However, a material with a more or less isotropic structure cannot be produced by means of this method.
A cast structure possesses a strength which is equal to about 1/100th to 1/1000th of the theoretical possible strength for the metal, due to internal structural inhomogeneties. However, this cast structure is broken down when the material is hot-worked, whereby a homogeneous structure and thereby higher strength is obtained. In exceptional cases, the strength of the material can be increased through hot-working to 1/10th of the theoretically possible strength.
The above-described structural anisotropy can be avoided more or less completely through the use of powder metallurgical processes, where the molten metal is disintegrated (fragmented) by gas or water to a metal powder. The metal powder--which is macroscopically, and even for the most part microscopically, chemically homogeneous--is then compacted by means of e.g. hot extrusion or hot isostatic compression to bars (billets) which are thereby rid completely of macroflaws. The mechanical properties of the resultant sintered material are good and isotropic, so that mechanical working of the material is unnecessary. However, powder metallurgy methods for the production of such sintered bars entail the disadvantage that there is a risk of oxidation and contamination of the metal powder, which impairs the mechanical properties of the bar and thereby the products made from the bar, as well as the disadvantage that the methods are relatively very expensive in terms of the cost of bars and uncomplicated final products.
Recently, attempts have been made to combine into a single operation the gas atomization of the molten metal and the collection of the resultant rapidly-solidified grains (microingots) in a mold to form an ingot. Such an ingot can then be directly hot-forged into a finished, non-porous and structurally isotropic product. (Atomised Scrap Forms Low Cost Forgings. Metals and Materials Nov. 1975, pp 39 and 40.) But this method, like methods where powder is added to gases, entails a major disadvantage, namely that the disintegration medium--which is a gas, in some cases containing powder--must be supplied in relatively large quantities--around 800 liters per kg molten metal--which leads to difficult-to-control turbulence phenomena in and around the collection moulds for the solidified or semi-molten powder grains. These turbulence phenomena result in an uneven distribution of mass in the collection mould, greatly reducing yield. In order to obtain a higher yield, attempts have been made to locate the collection mould closer to the point of disintegration. If this is done, however, the disintegrated molten metal does not have time to solidify before it reaches the mould, which, in combination with the aforementioned turbulence phenomena, causes the particles to stick to the walls of the collection mould to an extent where yield is greatly reduced. Due to these difficulties only very small ingots have been produced by means of this method.
Thus, a method has not yet been known which can be used to make large ingots from a segregation-prone material which are more or less structurally isotropic, and which does not entail the above-mentioned disadvantages associated with the various production methods presently known.